As an aid in the diagnosis and treatment of disease, it is often desirable to monitor the pressures at various positions within a patient's body or adjacent internal organs. A particular example is the measurement of the intracranial pressure within a patient's skull, since such measurements provide an indication of abnormalities in perfusion pressure or fluid retention, and allow the effect of drugs on intracranial pressure to be accurately monitored for effective treatment.
Various types of sensors are now utilized to measure internal body pressures, including electrical pressure transducers which transmit an elctrical signal indicative of the pressure through transmission wires out of the patient to a recording device, and pressure sensing heads which expand or contract in response to the pressure within the patient and communicate through a tube from the patient to a remote transducer which converts the pressure within the tube to a signal which can be displayed to the operator. The use of an electrical transducer implanted within a patient carries the obvious risks of shocks and short circuits, as well as the noise and baseline drift problems associated with any electrical transducer of a size small enough to be implanted. Direct pressure transmitting systems suffer from a lack of accuracy because of the distance that the pressure head must be transmitted from the patient to the remote transducer. The connection of the implanted sensing head through a tube to an external transducer also presents the possibility of a rupture or leak which would release air into the patient and possibly provide a source of infection.
In another type of pressure sensing apparatus, air flow is directed to a pressure sensor which includes a diaphragm covering a cavity within the sensor body. The diaphragm meters flow through an orifice by restricting or closing the orifice; the flow of air through the sensor is thereby controlled to equalize the pressure on both sides of the diaphragm, allowing the pressure at the sensor to be accurately read by reading the pressure of the air flowing in the tube leading to the sensor. In such a system, as well as in those which use a pressure sensing head which transmits the pressure through a tube to an external transducer, there is a small but definite risk that air (or other gas being used as the transmission medium) may leak from either the tubes or the sensor into enclosed spaces within the patient. Even leaks which occur outside the patient are detrimental to accurate measurement of the pressures within the patient.
Any closed, gas circulating system, in addition to those described above for physiological monitoring, may be subject to leakage which can effect system performance. A leak within such closed systems can result in abnormal pressures within the apparatus and erroneous data.